The use of orthopedic implants is the result of deterioration of human bone structure, usually because of various degenerative diseases, such as osteoarthritis. In recent years, a variety of implantable orthopedic devices had been developed. Typically, the failed bone structure is replaced with an orthopedic implant that mimics the structure of the natural bone and performs its functions.
Orthopedic implants are constructed from materials that are stable in biological environments and withstand physical stress with minimal deformation. Such materials must possess strength, resistance to corrosion, biocompatibility, and good wear properties. Also, the implants include various interacting parts, which undergo repeated long-term physical stress inside the body.
A breakdown of a permanently installed implant leads to pain, limitation on the range of motion, and may require a replacement of the implant. For these reasons, among others, the bone/implant interface and the connection between various parts of the implant must be resistant to breakdown. It is especially important since installation of an orthopedic implant often involves extensive and difficult medical procedure, and therefore replacement of the installed implant is highly undesirable.
The requirements for the useful life of the implant continue to grow with the increase in the life expectancy. The strength and longevity of implants in large part depend on the bone/implant interface. Various methods of connection are known in the art. For example, a hip joint is a ball-in-socket joint, and includes a rounded femoral head and a cup-like socket (acetabular cup) located in the pelvis. The surfaces of the rounded femoral head and the acetabular cup continually abrade each other as a person walks. The abrasion creates stress on the bones that bear the acetabular cup and the femoral head. If the femoral head or the acetabular cup is replaced with an implant, this stress must be well tolerated by the implant's bearing surfaces to prevent implant failure.
FIG. 1 shows a typical hip replacement system that includes an acetabular cup prosthetic assembly 10 and a femoral prosthesis 20. Generally, the acetabular cup implant 10 includes a bone interface shell 11 and a socket bearing insert 12. The femoral prosthesis 20 includes a femoral stem 21 and a femoral head in the form of a ball 22, which moves inside the socket insert 12 of the acetabular cup implant 10. The femoral ball 22 usually has a polished surface to maintain a LOW friction interface with the surface of the socket insert 12 of the acetabular cup 10. The stem section 21 is inserted into the interior of the femur and may have a bone interface surface 26.
The socket insert 12 is usually made from a plastic material such as polyethylene or ultra high molecular weight polyethylene (UHMWPE), but may be of any biocompatible material that has sufficient strength and wear resistance to withstand the pressures and abrasive nature of the joint. The socket insert 12 is typically held in the shell 11 by a series of locking grooves or notches. In turn, the complete acetabular cup implant 10 may be attached to the patient's pelvis by a series of locking grooves, pins or screws 29. Alternatively, the acetabular cup implant 10 may be press-fit by being driven into the patient's acetabulum with a proper impaction tool without the fixing pins in situations where patient-related criteria are met. This method avoids the use of bone cement. The shell 11 is typically made from a metal such as titanium or cobalt-chrome alloy, and has a bone interface surface 16.
In use, the bone interface surfaces 16 and 26 must bear a significant lateral and axial stress. The increased requirements for useful life of the implant make it especially important that these surfaces tolerate such stress. The prior art takes several approaches to this problem.
Thus, the entire acetabular cup implant 10, including both the shell 11 and the socket insert 12, may be cemented to the acetabulum or the cup may be produced as a single piece from ultra high molecular weight polyethylene and anchored into the acetabulum with bone cement. Another way to improve the longevity of orthopedic implants is to provide a porous bone interface surface to receive ingrowth of bone tissue thereby binding the natural bone to the implant. The bone ingrowth into the voids of the porous bone interface layer provides skeletal fixation for the implants used for replacement of bone segments. In addition to lateral and axial strength enhancement, the bone ingrowth improves biocompatibility of the implant and is even believed by some to promote positive biochemical changes in the diseased bone. To implement this approach, it is important to develop methods of constructing porous outer layers on the bone interface surfaces of implants.
Orthopedic implants with porous bone interface surfaces have been studied extensively over the last twenty years. It has long been known that the success in facilitating the ingrowth is related to the pore characteristics of the bone interface surfaces, such as pore size and pore volume. For example, it is known that the bone ingrowth may be almost entirely non-existent if the porous layer has pore sizes of less than 10 μm, and that pore sizes greater that 100 μm facilitate the ingrowth.
In view of the strength and longevity requirements, the implants are typically made of biocompatible metals, such as titanium or cobalt-chrome alloy. Thus, one of the challenges is to provide metallic orthopedic implants having porous metallic bone interfaces with high porosity. Another challenge is to provide an integrated bond between the porous layer and the underlying solid substrate, such as the surface 16 and the bulk of the shell 11, respectively, of the acetabular cup implant 10 shown in FIG. 1.
Certain orthopedic implants having porous bone interface surfaces, and related methods of making such implants have been patented. U.S. Pat. No. 5,282,861 describes an open cell tantalum structures for bone implants having pore volume of from 70 to 80%. The open cell tantalum structures of the '861 patent are formed by chemical vapor deposition of tantalum on a carbon skeleton. The resulting structures have a carbon core and a tantalum outer surface.
U.S. Pat. No. 6,087,553 describes tantalum/polyethylene composites suitable for use in orthopedic implants. The composites have a pore volume of 50 to 90%. The implants produced from the composites of the '553 patent are not modular and not metal-backed.
In general, methods of producing high pore volume metals are known in the art. U.S. Pat. No. 5,976,454 describes a process for producing nickel foam for use in making battery electrodes. The porosity of the foam is over 90%, but it is produced by a method that is in many respects not suitable for producing foams of biocompatible metals typically used in making implants, such as tantalum or titanium.
U.S. Pat. No. 5,926,685 describes a method of forming an implant having a porous outer surface by using an organic binder compound to enhance the binding between the porous surface layer and the implant. The binder and metal particles that would form the porous layer are mixed and the mixture is placed in contact with a solid surface of the metallic implant. Then, the particles (pre-cursor of the porous layer) are bound to each other and to the solid surface of the implant via a sintering process. The '685 patent does not describe production of a metal foam as a pre-cursor to the porous layer. Also, the '685 patent does not describe the porosity of the porous layer.
Therefore, there exists a continuing need for implantable medical devices, especially orthopedic implants, having porous surfaces, blocks, layers or other porous structures for interfacing with bones and/or other tissue, with the porous structures having a variety of desirable characteristics, including high porosity, uniform pore size, and high strength.